Pointer position in screen pixels.
Candidate reference features with their snapping rules.
Project a data coordinate to screen pixels.
OptionalsearchExtent: Extent
Optional data-coordinate box around the pointer (covering the largest tolerance). Features outside it are skipped without projecting them.
The nearest candidate, or null when nothing is within tolerance.
Find the nearest reference vertex (and optionally edge point) within each source's pixel tolerance of a pointer position.